Post-collisional magmatism in NE Australia during Mesoproterozoic supercontinent Nuna: Insights from new zircon U[sbnd]Pb and Lu[sbnd]Hf data

Abstract

The geodynamic regimes that operated during the Mesoproterozoic amalgamation of the first supercontinent on Earth, Nuna, remain poorly understood. Palaeogeographic Nuna reconstructions indicate that NE Australia and NW Laurentia were adjacent at its core—the ca. 1600 Ma collision between the Australian upper-plate (i.e., the Mount Isa Inlier) and the western Laurentian lower-plate (i.e., the Georgetown Inlier) was followed by a post-collisional, extensional regime associated with widespread low-pressure (LP) and high-temperature (HT) metamorphism and the formation of voluminous granitic melts. To better understand the tectonic regimes and crustal processes that operated during this Proterozoic orogenic system, new coupled U[sbnd]Pb and Lu[sbnd]Hf analyses from Mesoproterozoic granites in NE Australia are used to examine the ca. 1560 to 1540 Ma magmatism that occurred during the waning stages of Nuna amalgamation. LT, hydrous, S-type granites from the Georgetown Inlier yield crystallisation ages of ca. 1560–1550 Ma, whereas 100 km west of it, a newly identified Proterozoic Cudgee Creek Granite yield a crystallisation age of ca. 1540 Ma. The Cudgee Creek Granite is a HT, hybrid I−/A-type granite, reflecting a transitional geochemical trend towards the A-type granites in the Mount Isa Inlier further to the west. Zircon Lu[sbnd]Hf isotopic composition supports this geochemical trend with more radiogenic εHf(t) values from east to west, reflecting an increase in mantle input westward. The younging and the geochemical trend from the Georgetown Inlier S-type (ca. 1560–1550 Ma) to the I−/A- Cudgee Creek granite (ca. 1545 Ma) and the A-type granites in the Mount Isa Inlier (ca. 1530–1490 Ma) indicate progressive lithospheric thinning of the upper plate due to slow rollback of the Laurentian lower plate. These results suggest ongoing subduction drove hot and slow orogenesis involving slab rollback that was active for at least 100 m.y after the collisional stage.